Forty dry bean genotypes were tested in the semi-arid south coast of Puerto Rico to determine the effect of inoculation of experimental plots with bean plant debris highly infected with Macrophomina phaseolina. Analysis of variance indicated that significant differences (P=0.05) were found among the genotypes for disease severity indexes. The ashy stem blight infection increased approximately 40% between pod set and physiological maturity. A highly significant correlation (r=0.66**) was found between the disease severity ratings taken at 70 and 90 days after planting. Among 40 genotypes evaluated, Cuarentena and RIZ 44 showed high levels of susceptibility, whereas 8437-22 was the most resistant genotype to M. phaseolina.